365 
JAA, opy Mew Se 
The PRESIDENT (PROFESSOR BABINGTON) in the Chair. 
On the Centre of Motion of the Eye. By Pror, CuerK 
? MaAxweELu. 
The series of positions which the eye assumes as it is rolled 
horizontally have been investigated by Donders (Donders and 
Doijer, Derde Jaarlijksch Verslag betr. het Nederlandsch Gas- 
thus voor Ooglijders. Utrecht, 1862), and recently by Mr J. L. 
Tupper (Proc. R. S., June 18, 1874). The chief difficulty in 
the investigation consists in fixing the head while the eyeball 
moves. The only satisfactory method of obtaining a system of 
co-ordinates fixed with reference to the skull is that adopted 
by Helmholtz (Handbuch der Physiologischen Optik, p. 517), 
and described in his Croonian Lecture. 
A piece of wood, part of the upper surface of which is 
covered with warm sealingwax, is placed between the teeth and 
bitten hard till the sealingwax sets and forms a cast of the 
upper teeth. By inserting the teeth into their proper holes in 
the sealingwax the piece of wood may at any time be placed in 
a determinate position relatively to the skull. ; 
By this device of Helmholtz the patient is relieved from 
the pressure of screws and clamps applied to the skin of his 
head, and he becomes free to move his head as he likes, pro- 
vided he keeps the piece of wood between his teeth. 
If we can now adjust another piece of wood so that it shall 
always have a determinate position with respect to the eyeball, 
we may study the motion of the one piece of wood with respect 
to the other as the eye moves about. 
For this purpose a small mirror is fixed to a board, and a 
dot is marked on the mirror. If the eye, looking straight at 
the image of its own pupil in the mirror, sees the dot in the 
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